Multiple negative regulatory mechanisms exist that act to dampen the immune response to immune-based treatments. Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of early myeloid cells that accumulate in the blood and tumors of patients with cancer. Their numbers correlate with tumor burden and are predictive of overall survival. MDSC have been shown to reside in the peripheral blood, lymphoid tissue, and tumor tissue of mice in a number of experimental models. MDSC can inhibit the proliferation and cytotoxic activity of T cells in tumor-bearing animals through multiple mechanisms, and studies in murine models indicate that disruption of MDSC function can reverse immune tolerance to tumor antigens, stimulate anti-tumor immune responses, and induce tumor regressions. We have investigated the inhibitory effects of MDSC on human immune cells and found that they inhibit cytokine signal transduction within innate immune effector cells. Our murine experiments demonstrate that the abundant MDSC in tumor-bearing mice produce large amounts of nitric oxide which leads to increased nitration of tyrosine residues on signal transduction proteins and impaired responsiveness of immune effector cells to stimulatory signals. Also, we have recently been able to show that MDSC markedly inhibit the ability of natural killer (NK) cells to lyse monoclonal antibody (mAb)-coated tumor cells via effects on signal transduction downstream of the receptor for the constant (or